Strong Site Preference Research Articles

Activating Ru in the pyramidal sites of Ru2P‐type structures with earth‐abundant transition metals for achieving extremely high HER activity while minimizing noble metal content

AbstractRational design of efficient pH‐universal hydrogen evolution reaction catalysts to enable large‐scale hydrogen production via electrochemical water splitting is of great significance, yet a challenging task. Herein, Ru atoms in the Ru2P structure were replaced with M = Co, Ni, or Mo to produce M2−xRuxP nanocrystals. The metals show strong site preference, with Co and Ni occupying the tetrahedral sites and Ru the square pyramidal sites of the CoRuP and NiRuP Ru2P‐type structures. The presence of Co or Ni in the tetrahedral sites leads to charge redistribution for Ru and, according to density functional theory calculations, a significant increase in the Ru d‐band centers. As a result, the intrinsic activity of CoRuP and NiRuP increases considerably compared to Ru2P in both acidic and alkaline media. The effect is not observed for MoRuP, in which Mo prefers to occupy the pyramidal sites. In particular, CoRuP shows state‐of‐the‐art activity, outperforming Ru2P with Pt‐like activity in 0.5 M H2SO4 (η10 = 12.3 mV; η100 = 52 mV; turnover frequency (TOF) = 4.7 s−1). It remains extraordinarily active in alkaline conditions (η10 = 12.9 mV; η100 = 43.5 mV) with a TOF of 4.5 s−1, which is 4x higher than that of Ru2P and 10x that of Pt/C. Further increase in the Co content does not lead to drastic loss of activity, especially in alkaline medium, where, for example, the TOF of Co1.9Ru0.1P remains comparable to that of Ru2P and higher than that of Pt/C, highlighting the viability of the adopted approach to prepare cost‐efficient catalysts.

Cr dopant regulating d-orbital electronic configuration of NiFe spinel oxide to improve oxygen evolution reaction in Zn-air battery

Significant advancements towards enhancing the catalytic activity of spinel ferrites in oxygen evolution reaction (OER) have been achieved, however, further studies are required on this regard. In this study, Cr-doped spinel ferrite (NiFe2O4) embedded in N-doped C (Cr-NiFe2O4@NC) was synthesized via a high-temperature treatment using an NiFe Prussian blue analog as a precursor and K2Cr2O7 as a Cr source. Owing to the strong octahedral site preference energy, Cr3+ ions were successfully doped into octahedral sites and tailored the electronic configuration of NiFe2O4, as confirmed by XPS. The Cr-substituted Cr-NiFe2O4@NC exhibited the overpotential of 241 mV at 10 mA cm−2, which was smaller than that of NiFe2O4@NC (266 mV) at the same current density. Meanwhile, the Tafel slope of Cr-NiFe2O4@NC (65.32 mV dec−1) was also lower than that of NiFe2O4@NC (86.96 mV dec−1). Therefore, the Cr substitution can improve the electrocatalytic OER performance of NiFe2O4@NC. In particular, an assembled rechargeable Zn-air battery using Cr-NiFe2O4@NC as the cathode catalyst afforded a high power density (61.1 mA cm−2) together with a high specific capacity (740 mA g−1). Amazingly, it could power light-emitting diodes, demonstrating its real-world application. Density functional theory calculations revealed that Cr ion doping led to electron transfer in NiFe2O4, resulting in a density of electronic states close to the Fermi level, which probably regulated the adsorption/desorption behaviors of oxygenate intermediates (*O and *OOH), thus promoting the reaction kinetics of the OER. This study provides a helpful basis for the development of spinel oxides with high electrocatalytic OER activity and durability via cation doping.

Mg and Al-induced phase transformation and stabilization of Ga2O3-based γ -phase spinels

Ga2O3 films were deposited on (100) MgAl2O4 spinel substrates at 550, 650, 750, and 850 °C using metal-organic chemical vapor deposition and investigated using x-ray diffraction and transmission electron microscopy. A phase-pure γ-Ga2O3-based material having an inverse spinel structure was formed at 850 °C; a mixture of the γ-phase and β-Ga2O3 was detected in films grown at 750 °C. Only β-Ga2O3 was determined in the films deposited at 650 and 550 °C. A β- to γ-phase transition occurred from the substrate/film interface during growth at 750 °C. The growth and stabilization of the γ-phase at the outset of film growth at 850 °C was affected by the substantial Mg and Al chemical interdiffusion from the MgAl2O4 substrate observed in the energy-dispersive x-ray spectrum. Atomic-scale investigations via scanning transmission electron microscopy of the films grown at 750 and 850 °C revealed a strong tetrahedral site preference for Ga and an octahedral site preference for Mg and Al. It is postulated that the occupation of these atoms in these particular sites drives the β-Ga2O3 to γ-phase transition and markedly enhances the thermal stability of the latter phase at elevated temperatures.

The ordering behavior of Co3Al-based γ′ phase with L12 structure predicted by the thermodynamic model with support of first-principles calculations

The fundamental aspects of temperature- and composition-dependent site preference of alloying elements on sublattice of Co 3 Al-based γ′ phase with L1 2 structure have not been well understood, which hinders the design of advanced Co-based high-temperature alloys. In this work, the temperature-dependent and composition-dependent site preferences of the binary, ternary, and quaternary Co 3 Al-based γ′ phase alloyed with M i were studied using a two-sublattice thermodynamic involving site occupying fractions (SOFs). The SOFs were calculated using a thermodynamic software package based on a thermodynamic database established in this work, where the temperature-dependent thermodynamic data were obtained using first-principles calculations based on density-functional theory (DFT) and density-functional perturbation theory (DFPT). The additional alloying elements M i added represent the transitional metals Cr, Cu, Fe, Mn, Mo, Ni, Re, Ta, Ti, V, or W (arranged in alphabetical order). The results show that there is an obvious order-disorder transition for the stoichiometric and non-stoichiometric binary Co 3 Al as well as in ternary and quaternary Co 3 Al-based systems alloyed by adding additional transition metals around the solvus temperature. For the stoichiometric Co 3 Al γ′ phase, the critical temperature is about 1073 K, accompanied with the site configuration changes from (Al) 1a (Co) 3c at room temperature to (Al 0.2590 Co 0.7409 ) 1a (Al 0.2469 Co 0.7530 ) 3c at 1273 K, while this order-disorder transition was not found in the Ni 3 Al-based γ′ phase, a counterpart of Co 3 Al-based γ′ phase. When x Co / x Al is over 3:1, for example, the non-stoichiometric Co 3 Al–based binary γ′ phases 78Co-22Al show continuously order-disorder transition characteristics with a broadened transition temperature range. For the γ′ phase with the composition 78Co-26Al-4M i which is normalized as 72.22Co-24.08Al-3.7M i (at.%), where we kept x Co / x Al = 3:1 to study the ordering behaviors of the additional alloying transition metals M i conveniently. It is revealed that Cu, Fe, Mn, and Ni atoms always prefer the 3c sublattice (Co site), and Mo, Ta, Ti, and W atoms always prefer the 1a sublattice (Al site) in the whole temperature range, while the site preferences of Cr, Re, and V atoms are affected by the heat treatment temperature. For example, when the heat treatment temperature is lower than 700 K, Cr and V atoms randomly occupy 1a and 3c sublattice, and Re atoms prefer to 3c sublattice, while when the heat treatment temperature is higher than 1273 K, Cr atoms prefer 3c sublattice, V atoms prefer to 1a sublattice, and all Re atoms occupy 3c sublattice exclusively. The site preference of the quaternary system with a selective composition 78Co-26Al-2M 1 -2M 2 , which is normalized as 72.22Co-24.08Al-1.85M 1 -1.85M 2 (at. %) has been also investigated. The additional alloying transition metals M i added modulate the order-disorder transition characteristics of Co 3 Al-based γ′-precipitate with different amplitudes. The order-disorder transition temperatures for some selected alloys were verified experimentally using DSC. Based on the causal relationship of structure and mechanical properties, according to the modulating amplitude of the order-disorder transition behavior, we recommend that the additional alloying elements added such as Cu, Mn, and Mo atoms make the mechanical properties degrade gradually, and thus the structural components avoid sudden damage, which is beneficial for the safety of the engine component at high operating temperature. Fig. The temperature and composition-dependent site occupying fractions (SOFs) of the alloying element in Co 3 Al-based γ′ phase, where stoichiometric, non-stoichiometric binary Co 3 Al, ternary and quaternary systems by adding additional alloying elements were predicted, meanwhile, the atom distributions of the full ordered configuration and the disordered configuration at high temperature are presented graphically. It is revealed that there is a sudden order-disorder transition (also called first-ordered phase transition) in the Co 3 Al-based γ′ phase with L1 2 structure, meanwhile, Cu, Fe, Mn, and Ni atoms always prefer 3c sublattice (Co site), and Mo, Ta, Ti, W atoms always prefer 1a sublattice (Al site) in the whole temperature range, while the site preference of the alloying elements such as Al, Co, Cr, Re, or V is affected by the heat treatment temperature. The additional alloying elements M i added modulate the order-disorder transition characteristics of Co 3 Al-based γ′-precipitate with different amplitudes. • Co 3 Al-based γ′ phases show strong site preference and order-disorder transition, which were verified using DSC. • The transition temperature is about1000 K, while is modulated and broadened by additional elements. • Cu, Fe, Mn, and Ni always prefer 3c sublattice, while Mo, Ta, Ti, and W always prefer 1a sublattice in the whole temperature range. • The site preferences of Al, Co, Cr, Re, and V are temperature-dependent.

Enhanced Lithium Storage in Micrometer‐Scale Tungsten Bronze Mo3Nb2O14 by Molybdenum Reduction and Oxygen Deficiency

AbstractTungsten bronze transitional metal oxides are potential candidate anode material for lithium‐ion batteries (LIBs) owing to their open multichannel frame structure facilitating lithium transport and storage. Herein, the molybdenum reduction and oxygen deficiency are enhanced in micrometer‐scale Mo5O14‐type tungsten bronze structure Mo3Nb2O14−x (V‐MNO) that is prepared by a solid‐state reaction in vacuum. Neutron powder diffraction data indicate that oxygen vacancies are located at both three‐coordinated (µ3) oxygen sites around filled pentagonal rings and normally two‐coordinated (µ2) oxygen sites. As anode material for LIBs, benefiting from the increased Mo reduction, facilitating the electronic transport and oxygen vacancies without strong site preferences, widening the intratunnel, and opening up the intertunnel migration paths for lithium ions, V‐MNO displays enhanced electrochemical properties with an initial discharge capacity of ≈322 mAh g−1, a charge capacity of ≈274 mAh g−1, and a reversible capacity of ≈147.2 mAh g−1 (at 400 mA g−1) after 200 cycles. The LiCoO2//V‐MNO full cell shows a discharge capacity of 145.4 mAh g−1 after 100 cycles at 100 mA g−1. These results underline significance of controlling defect chemistry on the cationic reduction and oxygen vacancies in micrometer‐scale tungsten bronze transition metal oxides as an effective strategy for enhancing their storage performance as anode materials.

Doped MgAl2O4 semiconductor host nanopowders for tunable primary color emissions for application as white LEDs

The emission of white light from hosts having a single phase and the compromised applicability of a very inert, chemically stable, and structurally diverse MgAl2O4 led to a quest for improving the structural, optical, thermal and electrical properties for more innovative applications. A slight modification in lattice site occupation can lead to antisite defects which alter the physical properties of the materials. The presence of larger alkaline earth Sr2+ cations at the T-sites have been found to improve the behavior of the activator Mn2+ cations which are substituted for Mg2+ at T-sites. In the current study, a series of Mn2+ doped derivatives having a general formula of Mg1−x−0.3Mn x Sr0.3Al2O4 (x = 0.1–0.5) were synthesized using the chemical co-precipitation method. Due to the larger radius of Sr2+ cations, the lattice strain was observed in the cubic crystal structure of magnesium aluminate spinel (MAS). A decrease in the bandgaps of the doped samples indicates the formation of defect states within the bandgap of MAS. Apart from the bandgap transitions, the capture of electrons at oxygen vacancies is also observed in the UV/Vis spectra. The strong tetrahedral site preference of Mn2+ is altered by the presence of Sr2+, hence, some octahedral site occupation of Mn2+ in the lattice is achieved. The antisite defects along with the occupation of both T- and O-sites by Mn2+ resulted in novel emission bands centered at 464 nm (blue), 515 nm (green), and 621 nm (red) at λ ex of 380 nm. The emission of the primary colors in a simple lattice with cost-effective and in-toxic constituents can be a possible alternative to the costly rare-earth ions doped complex materials in use to-date.

Fuzzy logic: about the origins of fast ion dynamics in crystalline solids.

Nuclear magnetic resonance offers a wide range of tools to analyse ionic jump processes in crystalline and amorphous solids. Both high-resolution and time-domain , , , NMR helps throw light on the origins of rapid self-diffusion in materials being relevant for energy storage. It is well accepted that ions are subjected to extremely slow exchange processes in compounds with strong site preferences. The loss of this site preference may lead to rapid cation diffusion, as is also well known for glassy materials. Further examples that benefit from this effect include, e.g. cation-mixed, high-entropy fluorides , Li-bearing garnets () and thiophosphates such as . In non-equilibrium phases site disorder, polyhedra distortions, strain and the various types of defects will affect both the activation energy and the corresponding attempt frequencies. Whereas in () cation mixing influences F anion dynamics, in () the potential landscape can be manipulated by anion site disorder. On the other hand, in the mixed conductor cation-cation repulsions immediately lead to a boost in diffusivity at the early stages of chemical lithiation. Finally, rapid diffusion is also expected for materials that are able to guide the ions along (macroscopic) pathways with confined (or low-dimensional) dimensions, as is the case in layer-structured or . Diffusion on fractal systems complements this type of diffusion.This article is part of the Theo Murphy meeting issue ‘Understanding fast-ion conduction in solid electrolytes’.

Exposure to Microbial Metabolite Butyrate Prolongs the Survival Time and Changes the Growth Pattern of Human Papillomavirus 16 E6/E7-Immortalized Keratinocytes in Vivo

Human papillomavirus (HPV) is a ubiquitous human pathogen that can be cleared by host immunity. Nonetheless, a small percentage of the patients develop persistent infection with oncogenic HPV, which poses an increased risk of developing HPV-associated malignancy. Although cell-mediated immunity is a known systemic factor, local factors that influence persistent HPV infection have not been fully investigated. HPV-related head/neck cancers have a strong site preference for the oropharynx, suggesting the existence of unique local factors that promote HPV-induced oncogenesis. The human oropharynx often harbors anaerobic bacteria that produce a variety of byproducts, including butyrate. Because butyrate is a potent epigenetic modulator, it could be an environmental factor influencing the development of HPV-positive oropharyngeal malignancy. In this study, we showed that butyrate treatment changed the property of HPV16 E6/E7-immortalized keratinocytes. In vitro, the treatment increased the cells' migration ability, slowed the growth, and increased the genotoxic resistance. When implanted in the syngeneic mice, the treated keratinocytes survived longer and exhibited a different growth pattern. The survival advantage obtained after butyrate exposure may increase the susceptibility of HPV-infected oropharyngeal keratinocytes to further malignant transformation. These results suggest that fermentation products of tonsillar bacteria may play an important role in the long-term persistence of high-risk HPV infection, which is a critical risk factor for developing HPV-positive oropharyngeal malignancy.

(Invited) Fast Ion Dynamics in Solids As Probed By NMR: Disorder, Dimensionality and Correlation Effects

NMR offers plenty of tools to analyze ionic jump processes in crystalline and amorphous solids at atomic scale. Both high-resolution and time-domain 1H(2H), 6,7Li, 19F, 23Na NMR helps shedding light on the origins of rapid self-diffusion in materials being relevant for energy storage.It is well accepted that in materials with strong site preferences, such as LiAlO2, the Li+ ions are subjected to extremely slow exchange processes. The loss of this site preference, as it is the case in glasses and some nanocrystalline phases, may, however, lead to rapid cation diffusion. Examples that benefit from this effect include cation-mixed, high-entropy fluorides ((Ba,Ca)F2) and LiTi2(PS4)3. In general, in non-equilibrium phases site disorder, polyhedra distortions, strain and the various types of local defects will affect both the migration activation energy and the corresponding Arrhenius pre-factor. Whereas in (Me,Ca)F2 (Me = Ba, Pb) cation mixing influences F anion dynamics, in Li6PS5X (X = Br, Cl, I) the potential landscape can be manipulated by anion site disorder and the introduction of extrinsic defects. In some cases, this leads to rather flat landscapes enabling the ions to have access to concerted displacement processes, as is seen in LLZO-type garnets. On the other hand, in the mixed conductor Li4+x Ti5O12 cation-cation repulsions immediately lead to a boost in Li+ diffusivity at the early stages of chemical lithiation. For Na-bearing closo-borates, the influence of rotational anion dynamics is usually considered to explain long-range cation transport. Recent examples of our group also include Li- and Na-conducting MOFs, for which, as in the case of the closo-borates and the famous Na-β''-alumina, transitions from correlated to uncorrelated motion are observed at sufficiently high temperature.Finally, rapid diffusion is also expected for materials that are able to guide the ions along (macroscopic) pathways with confined dimensions, as we have recently observed for layer-structured fluorides such as RbSn2F5 or MeSnF4 with their buried interfaces.

B2 ordering in body-centered-cubic AlNbTiV refractory high-entropy alloys

The phase stability of a bcc AlNbTiV high-entropy alloy at elevated temperatures is studied using a combination of machine-learning interatomic potentials, first-principles calculations, and Monte Carlo simulations. The simulations reveal a B2 ordering below about 1700 K, mainly caused by a strong site preference of Al and Ti. A much weaker site preference for V and Nb is observed, strongly affecting the alloys total configurational entropy. The underlying mechanisms of the B2 phase stability as opposed to the random solid solution are discussed in terms of a high persisting configurational entropy of the B2 phase due to strong sublattice site disorder.

Co-alloying effects on the site preference and the elastic properties of γ′-Ni3Al from first-principles calculations

The influences of co-alloying atom pairs Ta–X (X: Ta, Mo, W, Cr, Co, and Re) on the site preference and elastic properties of γ′-Ni3Al precipitate phase were studied using first-principles calculations, and the underlying mechanism for the strengthening effect is determined. It is found that the site preferences of Ta–X co-alloying atom pairs are in the sequence of Al–Al, Al–Ni, Ni–Al, and Ni–Ni sites except for Ta–Co. The atom pair Ta–Co does not display a strong site preference tendency. The elastic properties for the Al–Al configuration were examined, and the results were compared with those of single-alloying to investigate the co-alloying effect. It is shown that the elastic modulus and hardness are obviously improved by co-alloying. Moreover, the effect of co-alloying is stronger than that of single-alloying. The further analyses by the charge density difference and the electronic density of states reveal that the strong effect of the co-alloying is attributed to the enrichment of the charge density between X and its nearest-neighbor Ni, as well as the orbital hybridization between Ta/X-d and Ni-d which acts as covalentlike bonds. Our results can provide an in-depth insight into the physics in the strengthening effect of co-alloying elements on the mechanical properties of single crystal superalloys.

Structural and electronic properties of c-BN (111) surface with hydrogen/fluorine functionalization and nitrogen-based small-molecule adsorption

Using first-principles density functional theory calculations, we systematically investigate the structural and electrical properties of pure, hydrogen (H) and fluorine (F) functionalized polar (111) cubic boron nitride (c-BN) surface. In the absence of surface functionalization, the reconstructed B-terminated surface is energetically preferable. The hydrogenation is favorable for stabilizing N- and B-terminated surfaces, while the fluorination leads to the stable unreconstructed B-terminated structure due to strong site preference of F atoms. The reconstructed c-BN surface has magnetic characteristic, and the spin density distributions are mainly localized around the interlayer weak B–B bonds. The unreconstructed structures are nonmagnetic. Meantime, the adsorption behavior of nitric oxide (NO) and ammonia (NH3) molecules are investigated on the reconstructed c-BN surface. It is found that the adsorption of NO has a considerable effect on the energy levels near the Fermi level, while the energy levels of NH3 are located at the deep energy level below the Fermi level. Our theoretical results are helpful for understanding experimental phenomenon in practical applications and designing novel c-BN based molecule sensors.

Modulating the Structure and Magnetic Properties of ε-Fe2O3 Nanoparticles via Electrochemical Li+ Insertion.

ε-Fe2O3, a metastable phase of iron oxide, is widely known as a room-temperature multiferroic material or as a superhard magnet. Element substitution into ε-Fe2O3 has been reported in the literature; however, the substituted ions have a strong site preference depending on their ionic radii and valence. In this study, in order to characterize the crystal structure and magnetic properties of ε-Fe2O3 in the Fe2+/Fe3+ coexisting states, Li+ was electrochemically inserted into ε-Fe2O3 to reduce Fe3+. The discharge and charge of Li+ into/from ε-Fe2O3 revealed that Li+ insertion was successful. X-ray magnetic circular dichroism results indicated that the reduced Fe did not exhibit site preference. Increasing the Li+ content in ε-Fe2O3 resulted in decreased saturation magnetization and irregular variation of the coercive field. We present a comprehensive discussion of how magnetic properties are modified with increasing Li+ content using transmission electron microscopy images and considering the Li+ diffusion coefficient. The results suggest that inserting Li+ into crystalline ε-Fe2O3 is a useful tool for characterizing crystal structure, lithiation limit, and magnetic properties in the coexistence of Fe2+/Fe3+.

Dissociative Chemisorption of Methane on Stepped Ir(332) Surface: Density Functional Theory and Ab Initio Molecular Dynamics Studies

Surface reactions often have strong site preferences, in which undercoordinated atoms on the surface often dominate due to lower reaction barriers. Understanding the dynamical underpinning of such ...

Ba8CoNb6-xTaxO24 Eight-Layer Shifted Hexagonal Perovskite Ceramics with Spontaneous Ta5+ Ordering and Near-Zero τf.

Highly positive temperature coefficients of the resonant frequency (τf) of eight-layer hexagonal perovskites are hardly tunable, hindering their application as microwave dielectric resonators. Here, we show that a near-zero τf (∼0.48 ppm °C-1) can be achieved on eight-layer shifted hexagonal perovskite Ba8CoNb4Ta2O24, along with a permittivity εr of ∼30.6 and a Qf of ∼36400 GHz, through substitution of Ta for Nb, satisfying the resonator application requirement. The decrease in the τf of Ba8CoNb6-xTaxO24 takes place mainly through the decrease in the εr or temperature coefficient of permittivity, owing to the less covalent bonding and lower polarizability of Ta5+ compared to those of Nb5+. Synchrotron and neutron powder diffraction data, scanning transmission electron microscopy-high-angle annular dark field imaging, and atomic-scale X-ray energy dispersive spectroscopy elemental mapping reveal that Ta5+ cations in Ba8CoNb4Ta2O24 are naturally distributed in a partially ordered manner, showing a strong site preference on the Nb layers close to the central Co layer over the most out-of-center distorted Nb layers next to empty octahedral layers. This spontaneous Ta ordering in the niobate host is driven by different covalent bonding nature and second-order Jahn-Teller distortion extents of Ta5+ and Nb5+. The results demonstrate an effective way of substituting more ionic Ta5+ for Nb5+, which decreases the τf to near-zero values for eight-layer hexagonal perovskite niobate dielectrics.

Deciphering structural and magnetic disorder in the chiral skyrmion host materials Co x Zn y Mn z (x + y+ z = 20).

Co x Zn y Mn z (x + y + z = 20) compounds crystallizing in the chiral β-Mn crystal structure are known to host skyrmion spin textures even at elevated temperatures. As in other chiral cubic skyrmion hosts, skyrmion lattices in these materials are found at equilibrium in a small pocket just below the magnetic Curie temperature. Remarkably, Co x Zn y Mn z compounds have also been found to host metastable nonequlibrium skyrmion lattices in a broad temperature and field range, including down to zero field and low temperature. This behavior is believed to be related to disorder present in the materials. Here, we use neutron and synchrotron diffraction, density functional theory calculations, and dc and ac magnetic measurements to characterize the atomic and magnetic disorder in these materials. We demonstrate that Co has a strong site preference for the diamondoid 8c site in the crystal structure, while Mn tends to share the geometrically frustrated 12d site with Zn, due to its ability to develop a large local moment on that site. This magnetism-driven site specificity leads to distinct magnetic behavior for the Co-rich 8c sublattice and the Mn on the 12d sublattice. The Co-rich sublattice orders at high temperatures (compositionally tunable between 210 and 470 K) with a moment around 1μ B /atom and maintains this order to low temperature. The Mn-rich sublattice holds larger moments (about 3μ B ) which remain fluctuating below the Co moment ordering temperature. At lower temperature, the fluctuating Mn moments freeze into a reentrant disordered cluster-glass state with no net moment, while the Co moments maintain order. This two-sublattice behavior allows for the observed coexistence of strong magnetic disorder and ordered magnetic states such as helimagnetism and skyrmion lattices.

Structural insights into the mechanism of double strand break formation by Hermes, a hAT family eukaryotic DNA transposase.

Some DNA transposons relocate from one genomic location to another using a mechanism that involves generating double-strand breaks at their transposon ends by forming hairpins on flanking DNA. The same double-strand break mode is employed by the V(D)J recombinase at signal-end/coding-end junctions during the generation of antibody diversity. How flanking hairpins are formed during DNA transposition has remained elusive. Here, we describe several co-crystal structures of the Hermes transposase bound to DNA that mimics the reaction step immediately prior to hairpin formation. Our results reveal a large DNA conformational change between the initial cleavage step and subsequent hairpin formation that changes which strand is acted upon by a single active site. We observed that two factors affect the conformational change: the complement of divalent metal ions bound by the catalytically essential DDE residues, and the identity of the –2 flanking base pair. Our data also provides a mechanistic link between the efficiency of hairpin formation (an A:T basepair is favored at the –2 position) and Hermes' strong target site preference. Furthermore, we have established that the histidine residue within a conserved C/DxxH motif present in many transposase families interacts directly with the scissile phosphate, suggesting a crucial role in catalysis.

Magnetization reversal induced by Mn substitution in spinel chromite NiCr2O4

AbstractPolycrystalline Ni(Cr1−xMnx)2O4 (0.1 ≤ x ≤ 0.325) ceramic samples were studied through different protocols of dc magnetization measurements. The samples exhibit 2 kinds of magnetic compensation effects below the ferrimagnetic transition temperature TC. Remarkable magnetization reversal is observed between the 2 compensation temperatures Tcomp1 and Tcomp2, which is regarded as arising from the negative exchange coupling between the 2 magnetic sublattices at different crystallographic sites. The magnetization is reversed at TSR due to spin‐reorientation caused by magnetostructural coupling. The spin‐reorientation is supported by Mn substitution and TSR is increased to 96 K when x reaches 0.325. However, it is suppressed due to the strong ionic site preference and thus the magnetization is slightly increased in the negative direction of the magnetic field. Near the 2 compensation temperatures, tunable magnetic switching effects can be obtained just by changing the magnitude of the applied magnetic field. Moreover, both normal and inverse magnetocaloric effects were also demonstrated.

Residency and long-distance movements of sevengill sharks (Notorhynchus cepedianus) tagged in San Francisco Bay

BackgroundSevengill sharks are common inhabitants of estuaries and coastal areas and particularly abundant in San Francisco Bay (SFB). There is limited knowledge about the level of residency and migratory movements of this species. We describe the degree of residence of sevengill sharks in SFB and nearby locations, using ultrasonic tags and automated listening stations.ResultsSevengills showed a high degree of residency in SFB, particularly to the Golden Gate (GG) area where they have a strong site preference, both seasonally and inter-annually. Site fidelity was also the highest at the GG. In sharks with deployment times longer than 300 days, we found that > 80% of visits of all mature males were at GG, 80% of visits of a single mature female occurred there, > 85% of visits of most juvenile males, and > 70% of visits of most juvenile females. At GG, sharks moved into the range of the receiver day and night without a strong preference for a particular time of the day, which implies movements up and down the area day and night. At most other locations within the bay, sharks were present during daytime hours, while at Point Reyes during nighttime hours. Sharks moved into SFB during early spring and summer, and moved out of SFB to Point Reyes during late spring and fall. Three sevengill sharks migrated roughly 800 km southeastward along the coastline from SFO, one making three trips back and forth between the sites.ConclusionsThere was strong evidence of residency of sharks within SFB, and sharks repeatedly returned to the same region of the bay with some sharks making long-distance movements.

Explorations on TiOsX (X=B, C, N, O and Si) alloys for potential superhard materials from first-principle calculation

Using the first-principle calculations, we gained a deep insight of the structural, mechanical, electronic and thermal properties of the TiOs and TiOs-X (X=B, C, N, O and Si) compounds. The calculated results of formation enthalpy Hform and ternary transfer energy EXOs→Ti illustrate that O element exhibits strong Os site preference and all the compounds here are structurally stable. The results of mechanical properties confirm that there is no superhard character in those intermetallics which are mechanically stable. It is noted that the Ti7Os8Si possesses the best mechanical properties. All the compounds here show elastic anisotropy, and the Ti8Os7O exhibits the strongest elastic anisotropy, while the Ti8Os7Si strongly tends to be isotropic. There exists a mixture of covalent, ionic and metallic characters in these compounds. The covalent bond strength of Ti7Os8X is supposed to be better than that of the corresponding Ti8Os7X. Moreover, the minimum thermal conductivity kmin of these compounds are comparatively small and show dependence of directions, and they have potential thermal-insulating application in engineering.